1. Field of the Invention
This invention relates to a method and system for measuring a parameter of motor operation, such as rotor speed, rotor direction, back EMF magnitude and/or back EMF phase angle. In a particularly preferred embodiment, this invention relates to a method and system in which each of these parameters is measured for purposes of reconnecting a motor to a motor drive.
2. Description of Related Art
There are many instances in which it is desirable to measure one or more parameters of motor operation. Typical parameters of interest include rotor speed, rotor direction, back EMF magnitude, and back EMF phase angle. During normal motor operation, adequate assumptions about these parameters can often be made based on the control that is implemented (e.g., if particular speed is commanded in an open loop control scheme, it is often adequate to assume that the control scheme is maintaining the actual motor speed at the commanded speed). However, situations exist in which such assumptions are not adequate. This is the case, for example, when a motor drive becomes disconnected from a motor (meaning that the power supply to the motor is interrupted, not necessarily that the electrical connection between the motor drive and the motor is interrupted) and open loop control is no longer present. In this case, with no control present, it is difficult to make any assumptions about these parameters.
There are a variety of reasons why a motor drive may become disconnected from a motor. For example, there may be a sudden temporary power loss at the power source that supplies power to the motor and motor drive. Alternatively, it may simply be the case that there are times when it is not necessary to operate the motor, and so power is not supplied to the motor during these times.
The fact that the motor drive is disconnected from the motor does not prevent the motor from continuing to rotate. For example, if the motor is used in conjunction with a fan in an air conditioning system, a draft in the air conditioning system may drive the motor at an unknown speed and in an unknown direction. Similarly, if the motor is used in a conveyor system, the force of gravity acting on the motor by way of the conveyed articles and friction may drive the motor at an unknown speed and in an unknown direction.
When a motor drive becomes disconnected from a motor, it eventually becomes necessary to reconnect the motor drive to the motor. In order to perform the reconnection, it is desirable to determine the above-mentioned parameters, namely, rotor speed, rotor direction, back EMF magnitude and/or back EMF phase angle, before the motor drive is reconnected to the motor. Measuring these parameters is desirable because it allows the motor drive to be synchronized to the motor, thereby reducing transients at the moment of reconnection. For example, if the speed of the motor is not determined before reconnection, then the motor drive must assume an initial speed of zero when reconnecting to the motor. This often results in severe transients due to the difference between the frequency of the applied voltage and the frequency of the motor-induced back EMF. The transients are especially severe when the initial motor speed is high and when the motor is rotating in a reverse direction as compared to that commanded by the motor drive. If the current control circuitry or current limiting circuitry of the motor drive is not fast enough, the motor drive can fault due to an overcurrent condition. Additionally, when the motor operates as a generator (that is, when the frequency of the voltage applied to the motor is less than the motor speed), then the DC bus voltage may increase to unacceptable levels and cause damage to the power switches in the motor drive.
It is therefore desirable to determine the rotor speed, rotor direction, back EMF magnitude and back EMF phase angle to allow the motor drive to be synchronized to the motor when the motor drive is reconnected and thereby to reduce transients upon reconnection. Additionally, when performing a reconnection, it is desirable to measure these parameters in as little time as possible so that operation may continue as smoothly as possible to make the temporary disconnection as imperceptible as possible.
According to a preferred aspect of the invention, a method of reconnecting a motor to a motor drive comprises controlling a current which flows through the motor using a current regulator. The current regulator produces a voltage command output based on a current command input. A current is commanded at the current command input of the current regulator. Back EMF measurements are acquired at different instants in time by monitoring the voltage command output of the current regulator. Back EMF phase angles are determined for a plurality of the instants in time based on a respective plurality of the back EMF measurements. A frequency of the back EMF is determined based on the back EMF angles determined for the plurality of instants in time. The motor drive is reconnected to the motor based on the frequency of the back EMF.
Other objects, features, and advantages of the present invention will become apparent to those skilled in the art from the following detailed description and accompanying drawings. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the present invention, are given by way of illustration and not limitation. Many modifications and changes within the scope of the present invention may be made without departing from the spirit thereof, and the invention includes all such modifications.